The present invention relates to mobile radio communication systems such as a land mobile communication system, a mobile satellite communication system, a digital land mobile satellite communication system, a digital land mobile communication system and a digital mobile satellite communication system.
Conventionally, a transportable earth station of the mobile satellite communication system and a portable terminal of the terrestrial based mobile communication system are independent of each other, and the air interface between the satellite and the transportable earth station and the air interface between the base station and the portable terminal of the terrestrial based mobile communication system differ from each other. On this account, to use both of the mobile satellite communication system and the terrestrial based mobile communication system, it is necessary to prepare the transportable earth station for the mobile satellite communication system and the portable terminal for the land mobile communication system independently of each other. One possible method that has been proposed as a solution to this problem is to use a dual-mode mobile terminal which functions both as the transportable earth station for the mobile satellite communication system and the portable terminal for the land mobile communication system. In urban areas the dual-mode mobile terminal accesses a base station of the terrestrial based mobile communication system to communicate with a terrestrial based fixed network, whereas in areas outside the service area of the terrestrial based mobile communication system, such as sub-urban areas and hilly or mountaineous districts, it accesses the satellite to communicate with the terrestrial based fixed network. This permits communication from anywhere in the world through the use of just one dual mode mobile terminal.
The conventional systems have such problems as listed below.
(1) The mobile terminal for the land mobile communication and the transportable earth station for the mobile satellite communication system are required to stay within the areas where to be visible from the base station and the satellite, respectively. Accordingly, when these terminals are used indoors, the mobile terminal or transportable earth station must be brought to a place where it is visible from the base station and the satellite so as to receive signals from either one of them.
(2) In case of using the dual-mode terminal for mobile satellite and terrestrial based communications, it is necessary that the mobile terminal having a scale larger than that for an ordinary land mobile communication system be carried at all times even in the service area of the terrestrial based mobile communication system wherein no communications with the satellite are needed. Hence, the conventional dual-mode terminal conspicuously lacks ease of use.
(3) In case of using the mobile satellite communication system, it is necessary to hold the satellite visible from the transportable earth station at all times; hence, restrictions are imposed on the site of placement of the earth station.